The Available Reserved Space reports the number of reserve blocks remaining. The normalized value decreases from 100 as the percent availability is reduced. The threshold value for this attribute is 10 percent availability, so your SSD has reached the threshold value.
The information you provided does not provide enough data to determine the reason why it decreased in your SSD.
We would advise you to Download Intel® Solid-State Drive Toolbox version 3.3, and use the tool to update the firmware of your Intel® Solid State Drive 520 Series.
If the issue persists with the latest toolbox and firmware, please Contact Support and a support specialist will be able to provide further assistance about this.